[TASK] Add regression test for DecoratingPlantumlBinaryRenderer

Add unit tests to verify the decorator:
- Creates temp directory when missing before rendering
- Properly delegates to the inner renderer

Also changes constructor to accept DiagramRenderer interface
instead of concrete PlantumlRenderer to enable mocking in tests.

Relates: TYPO3-Documentation#1099

Author: CybotTM

packages/typo3-docs-theme/tests/unit/Renderer/DecoratingPlantumlBinaryRendererTest.php

@@ -0,0 +1,69 @@
+<?php
+
+declare(strict_types=1);
+
+namespace T3Docs\Typo3DocsTheme\Tests\Unit\Renderer;
+
+use phpDocumentor\Guides\Graphs\Renderer\DiagramRenderer;
+use phpDocumentor\Guides\RenderContext;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+use T3Docs\Typo3DocsTheme\Renderer\DecoratingPlantumlBinaryRenderer;
+
+use function is_dir;
+use function rmdir;
+use function sys_get_temp_dir;
+
+final class DecoratingPlantumlBinaryRendererTest extends TestCase
+{
+    private const TEMP_SUBDIRECTORY = '/phpdocumentor';
+
+    #[Test]
+    public function renderCreatesTempDirectoryWhenMissing(): void
+    {
+        $tempDir = sys_get_temp_dir() . self::TEMP_SUBDIRECTORY;
+
+        // Remove the directory if it exists to test creation
+        if (is_dir($tempDir)) {
+            @rmdir($tempDir);
+        }
+
+        // Skip if we can't remove it (contains files from other processes)
+        if (is_dir($tempDir)) {
+            self::markTestSkipped('Cannot remove temp directory - it contains files from other processes');
+        }
+
+        $innerRenderer = $this->createMock(DiagramRenderer::class);
+        $innerRenderer->method('render')->willReturn('<svg></svg>');
+
+        $renderContext = $this->createMock(RenderContext::class);
+
+        $decorator = new DecoratingPlantumlBinaryRenderer($innerRenderer);
+        $decorator->render($renderContext, 'A -> B');
+
+        self::assertDirectoryExists($tempDir);
+
+        // Clean up
+        @rmdir($tempDir);
+    }
+
+    #[Test]
+    public function renderDelegatesToInnerRenderer(): void
+    {
+        $expectedResult = '<svg>diagram</svg>';
+        $diagram = 'A -> B';
+
+        $renderContext = $this->createMock(RenderContext::class);
+
+        $innerRenderer = $this->createMock(DiagramRenderer::class);
+        $innerRenderer->expects(self::once())
+            ->method('render')
+            ->with($renderContext, $diagram)
+            ->willReturn($expectedResult);
+
+        $decorator = new DecoratingPlantumlBinaryRenderer($innerRenderer);
+        $result = $decorator->render($renderContext, $diagram);
+
+        self::assertSame($expectedResult, $result);
+    }
+}